There is ongoing progress toward widespread replacement of petroleum with bioethanol as an alternative fuel. Bioethanol is produced mainly through saccharification and fermentation of sugarcane or corn. In recent years, a technique is being developed to produce bioethanol from wood-based biomass and plant-based biomass (which are also referred to as cellulosic biomass) such as wood waste or unused portions of crops such as rice straw, which do not compete with foods and feeds.
In order to produce bioethanol from cellulosic biomass as a raw material by a conventional ethanol fermentation method, it is necessary to saccharify the cellulose. As a saccharification method, there are known a method using a concentrated sulfuric acid, a method using a diluted sulfuric acid and an enzyme, and a hydrothermal saccharification method; however, there are still many problems to be solved in order to produce bioethanol at a low cost.
Meanwhile, there is a method in which cellulosic biomass is converted to a mixed gas containing hydrogen and carbon monoxide, from which an alcohol is synthesized. With this method, an attempt is made to efficiently produce bioethanol from cellulosic biomass to which the application of alcohol fermentation is difficult. In addition, raw materials which can be used in this method are not limited to the wood-based biomass and the plant-based biomass, but also include various biomasses such as animal biomass derived from carcasses or feces of animals, garbage, waste paper and waste fiber.
Further, since a mixed gas of hydrogen and carbon monoxide is also derivable from sources other than petroleum, such as natural gas or coal, a method of synthesizing an alcohol from such a mixed gas has been studied as a technique to move away from petroleum dependency.
As a catalyst used for obtaining an oxygenate such as ethanol, acetaldehyde or acetic acid from a mixed gas of hydrogen and carbon monoxide, for example, there is known a catalyst comprising rhodium and an alkali metal which are supported on a silica gel carrier (see, for example, Patent Document 1). However, the technique of Patent Document 1 results in production of a large amount of oxygenates other than alcohols such as ethanol, whereby a long time and a large amount of energy are required for performing a step of separating alcohols.
For solving these problems, there is proposed a method for producing ethanol in which a mixed gas of carbon monoxide and hydrogen is brought into contact with a catalyst in a reaction apparatus which is filled with, at an upper layer thereof, a catalyst comprising rhodium supported on a carrier and is filled with, at a lower layer thereof, a catalyst comprising iridium and iron supported on a carrier or a catalyst comprising iridium, iron and rhodium supported on a carrier (see, for example, Patent Document 2).